Re: ata_check_status_mmio exception kernel panic

[Jeff Garzik <jgarzik@pobox.com>]

Alan Cox wrote:
>> port freeze during an interrupt while DMA is active causes a bus error 
>> to be thrown when the ata_check_status call fires.  I cannot reproduce 
>> this on x86.  I assume it handles the taskfile read error differently.
> 
> I hit this problem with the HPT343 (which locks the bus if you do this).
> For other devices where you get a PCI abort it is usually the case that
> x86 platforms ignore them while on many other platforms they produce
> exceptions.
> 
> In general I think we need to be sure all the core code paths stop DMA
> before poking in the taskfile (other than the basic IRQ check which we
> can't avoid)

I really think we are currently getting the ordering wrong, by calling 
ata_qc_complete() before we call the ->freeze() hook.

You can see sata_promise had problems with this, which led us to add DMA 
disabling in pdc_freeze()

But I think more generally, we seem to be missing a DMA-disable upon 
freeze, across a wide variety of controllers.

	Jeff